1. Field of the Invention
The present invention relates to revision knee implants and surgical orthopedic cutting instruments, and more particularly relates to an improved revision knee femoral prosthesis and corresponding improved cutting block apparatus for shaping a distal femur of a patient prior to implantation of the femoral prosthesis.
2. Description of Related Art
In patients who require a revision knee system implant, there typically is much bone loss, and thus a lack of normal bony reference points or landmarks for properly aligning the implant. In these cases, surgeons use the intramedullary canal of the femur as a landmark for positioning the prosthesis. If the femoral stem of the implant is not properly aligned with respect to the intramedullary canal in the anterior-posterior direction, there will be a gap between the natural femur and the femoral component at the anterior or posterior end.
A conventional knee prosthesis generally has a smooth continuous outer curvature that faces a corresponding component attached to the patient's tibia, and includes a femoral component, a femoral stem, a patellar component and a tibial component. The femoral component typically includes a pair of spaced apart condylar portions that articulate with the tibial component. Additionally, the femoral component usually has an intercondylar surface located between the two condylar portions. The femoral stem typically is connected to the intercondylar surface of the femoral component by means of a bolt that passes through a bore in the intercondylar surface. The femoral stem is, in use, inserted into a reamed intramedullary canal of the femur. The patellar component articulates with the anterior surface of the femoral component.
Conventional revision knee prostheses come in various sizes and shapes to compatibly match the anatomical constraints of a variety of individual patients. Additionally, to further match a particular patient's anatomy, the femoral stem of the prosthesis typically is set at an angle, from lateral to medial, to match the patient's valgus angle, which is the angle between the axis of the femoral shaft and an imaginary vertical line extending from the distal femur to the center of the femoral head.
Before implanting a revision knee prosthesis, the distal femur of the patient needs to be surgically cut to allow for the proper alignment of the prosthesis implant. A typical femoral component of a knee prosthesis provides five intersecting flat surfaces. One surface is adapted to engage the anterior cortical surface of the femur. Another surface is adapted to face the posterior surface of the femur. Still another surface is adapted to engage the distal end of the patient's femur. Additionally, a pair of chamfer surfaces form diagonally extending surfaces which form an interface between the distal surface and the respective anterior and posterior surfaces.
Before beginning a cutting operation, the surgeon needs to orient a cutting guide in anterior and posterior directions relative to the patient's femur and also relative to the valgus angle of the patient's femur. A surgeon needs to form five separate cuts on the patient's distal femur in order to prepare the distal femur to receive the femoral prosthesis. One of the problems encountered by the surgeon is the proper orientation of the cuts so that the prosthesis will fit the femur with the correct orientation.
Conventional knee prostheses typically present with several notable shortcomings. For example, the anterior-posterior position of the femoral stem usually is not adjustable or only adjustable in limited increments, in both cases resulting in poor alignment of the femoral prosthesis with respect to the intramedullary canal of the femur or an unbalanced flexion gap, either of which can lead to deterioration of the implant. One typical revision knee system provides incremental anterior-posterior positioning by means of three interchangeable screw heads which permit limited variation of the anterior/posterior position from the norm, i.e., 0 mm, limited to increments of +3 and −3 mm. In another revision knee system, the femoral stem is attached to the femoral component by means of a nut and bolt. If the bolt loosens, it can migrate into the knee joint, resulting in injury to the surrounding tissue and damage to or interference with the prosthesis. Additionally, such nut and bolt systems can introduce particulate matter into the knee joint which can cause infection or lead to inflammatory immune responses.
Thus, there exists a need for a prosthetic revision knee system that allows for an infinite adjustment of the anterior/posterior position of the femoral stem to suit a wide range of patient anatomies, that permits a proper alignment of the valgus angle of the femoral stem in order to accommodate the anatomical constraints of a variety of patients, that optimizes the positioning of a femoral component and/or femoral stem vis a vis the available bone; and that provides a femoral stem locking system that will not migrate into the knee joint or cause damage to the articular surface of the implant which may lead to infection or inflammatory immune responses.